It is well-known in the exploitation of an oil reservoir that, most of the time, no more than half of the oil initially present in the reservoir is extracted, or even less. The recovery by the primary means, that is, the use of extraction energy utilized from gases or liquids present in the subsoil and currently, initially, a certain pressure in the reservoir, allows for only extracting low percentages of the total oil present in the reservoir. To complete this primary recovery, one proceeds with a secondary recovery consisting in implementing what is called a “water drive” or “water flooding” production, that is, by injecting water in a well (injection well) at a location of the reservoir, so as to push the oil of the reservoir out of the subsoil, by at least one other called a “production well”.
To improve the secondary recovery by water drive, it is known to add surfactants to the injection water. This technique for optimizing the oil recovery by pushing injected water, to which surfactants may or may not have been added, also involves tracers that can be easily detected in the liquid, namely the injection water and push water at the exit of production wells. These tracers allow for measuring the arrival time, that is, the time which passes between the injection of the injection and push waters in the injection well(s) and the moment when these injection water and push water charged in tracers come out at the exit of one of several isolated production wells. From this arrival time, one can determine the volume of the reservoir that constitutes the oil reservoir. This is one of the most important parameters which can be determined by the use of tracer fluids, since it allows, on the one hand, adjusting the quantity of surfactant introduced in the injection and push waters, and on the other hand, evaluating the additional quantity of oil that can be expected following the implementation of this optimized method for recovering oil. As long as the fluid containing the tracer has been detected at the production well(s), the method of study for analyzing, monitoring, and optimally recovering oil requires the concentration of tracers in the fluid produced at the exit to be measured, continuously or not, so as to be able to plot the curves of tracer concentration as a function of the time or as a function of the volume of fluid produced.
Tracers in the injection water and push waters for oil reservoirs also enable detecting aberrations in the flow rates caused by pressure differentials in the reservoir, which are caused by factors other than the injection of injection and push waters and which hinder the performances.
The specifications of the tracers, usable in these injection and push waters for optimizing oil recovery comprise the following characteristics:                economical;        compatible with the fluids which are naturally present in the reservoir and with the oil-reservoir rock itself as well as with the fluids injected in the reservoir, namely, the injection and push liquids (waters);        easy qualitative and quantitative detection of the tracer no matter the materials present in the fluid at the exit of the production well. For example, an aqueous solution of sodium chloride cannot be used as tracer since most of the oilfields contain sea water, and thus a substantial quantity of sodium chloride, which makes detecting NaCl chloride used as tracer particularly difficult;        furtive tracer, that is, not easily absorbed in the solid medium traversed or eliminated from the tracing fluid, since with the analytical technique used, the tracer concentration in the fluids produced at the exit is determined and compared with that of the fluids injected in the injection well(s);        the tracer resists bacterial contamination, high temperatures and high pressures present in oil reservoirs;        the tracer has the ability to interact or not interact with the medium of the reservoir, namely, the geological media, oil-bearing or not;        access to a great number of tracers and different codings for possible simultaneous detections (several injection wells) or tracing test which are successive over time.        
Regarding the state of the art relating to such tracers for injection and push waters (tracing fluid) making it possible to probe oil reservoirs by diffusion between an injection well and a production well, one can cite the U.S. Pat. Nos. 4,231,426-B1 and 4,299,709-B1, which disclose aqueous tracer fluids comprising from 0.01 to 10% in weight of a nitrate salt associated with a bactericidal agent chosen among aromatic compounds (benzene, toluene, xylene).
The Canadian patent application CA 2 674 127-A1 relates to a method consisting in using a natural isotope of carbon 13 for identifying premature drilling of injection waters in oil wells.
In addition, there are a dozen families of molecules adapted and currently identified as tracers for injection waters in oil reservoirs. These molecule families are, for example, fluorinated benzoic acids or naphtalenesulfonic acids.
The tracer molecules which are known and used have a specific chemical/radioactive signature. These known tracers can be detected with great sensitivity. However, they have three major drawbacks:    their quantification requires a rather complex and expensive process and can be carried out only in a specialized center, often far away from productions sites;    these molecules, which are not numerous, do not allow for multi-labeling or repeated labelings;    some of these known tracers are caused to disappear due to their negative impact on the medium.
Furthermore, the site “Institute for Energy Technology” (IFE) has put online to date a PowerPoint presentation entitled SIP 2007-2009 “New functional tracers based on nanotechnology and radiotracer generators Department for Reservoir and Exploration Technology”. In particular, this document discloses the use of surface-modified nanoparticles as tracers for flow monitoring in oil reservoirs, oil wells and in process systems. More precisely yet, this presentation describes functionalized tracer nanoparticles comprising a Gd2O3-based core and a siloxane-based surface coating which is functionalized with additional molecules. The rare earth core and/or the additional molecules can emit light signals by fluorescence or radioactive signals.
However, to date, no study has shown the feasibility of the detection of particles for oil tracing. It seems to be even generally accepted that particles, due to the volume of space they fill, are filtered and strongly retained in the soil, thus preventing any reasonable use of their detection as tracing. It is actually based on this hypothesis that the above-mentioned PowerPoint presentation from IFE is based. In fact, this document does not make any reference to the detection of particles, but rather to the detection of a by-product or a residue related to the degradation of these particles.
In a completely different field, the French patent application FR 2 867 180-A1 describes hybrid nanoparticles comprising, on the one hand, a core made of rare earth oxides, optionally doped with rare earth or actinide or a mixture of rare earths or a mixture of rare earths and actinide, and, on the other hand, a coating around this core, said coating chiefly consisting of polysiloxane functionalized by at least one biological ligand grafted by covalent bonding. The core can be made of Gd2O3 doped with an amount of Tb3+ or by uranium and the polysiloxane coating can be achieved by making an aminopropyltriethoxysilane, a tetraethylsilicate, and a triethylamine react. These nanoparticles are used as probes for the detection, the monitoring, and the quantification of biological systems.
The French patent application FR 2 922 106-A1 relates to the same technical field and targets the use of these nanoparticles as radiosensitizing agents for making the radiotherapy more efficient. The size of these nanoparticles is comprised between 10 and 50 nanometers.